1. Field of the Invention
This invention relates to a continuous process for producing dry water insoluble metallic salts of higher fatty acids and more particularly to a continuous process employing blends of fatty acids, particularly mixtures of stearic and palmitic acids and bases such as zinc oxide or calcium hydroxide to prepare zinc or calcium salts.
2. Description of the Prior Art
Metallic salts of higher molecular weight fatty acids are commercially known as metallic soaps. Metallic soaps which are blends of principally stearic acid, also known as octadecanoic acid, and palmitic acid, also known as hexadecanoic acid, are offered commercially as metallic stearates. The major metallic stearates are calcium, zinc, aluminum and magnesium stearates. Barium, lead, copper, iron, silver and strontium stearates are of less commercial importance.
For many years, dry calcium stearate and zinc stearate have been used as internal lubricants in plastics, mold release agents for rubber, external lubricants for wire drawing and metal working, sanding sealers, flatting agents for varnishes and lacquers, cosmetic emulsifiers and aerosol dispensing aids. Aqueous dispersions of calcium and zinc stearates have also been used as paper coatings, dip coating for uncured rubber slabs or extrusions, waterproofing agent for cement, mold release and antitack agent for stored slabs, sheets and extruded articles.
There are numerous processes for the manufacture of calcium stearate, but these are batch operations based on wet fusion, dry fusion and precipitation processes. Zinc stearate is also produced by the wet fusion and precipitation processes.
The wet fusion process uses stearic acid and zinc oxide for zinc stearate or calcium hydroxide for calcium stearate. In this process, a heated batch reaction is carried out in an aqueous media containing an emulsifying agent. Stearates produced by the wet fusion process are used as 40-60% solids aqueous dispersions. This process reaction requires about two hours for completion and further processing is often required to produce uniform dispersions.
Dry fused calcium stearates are produced by adding stearic acid and calcium hydroxide to a batch reactor and heating. The reaction mass is heated to its melting point and maintained in a molten condition until reaction is complete. The batch is then cooled to solidify in order to permit its crushing and grinding to obtain the desired particle size. Stearates produced by the dry fusion process are characterized by their yellow color and dense particle. Because of these properties, use of dry fusion process stearates is restricted to the wire drawing and metal working industries.
Precipitated zinc and calcium stearates are characterized by their purity, white color, very fine particle size and low bulk density. The precipitation process involves:
(a) A batch saponification reaction employing sodium hydroxide and a fatty acid to produce an aqueous solution of sodium stearate.
(b) A batch precipitation reaction wherein calcium chloride (for calcium stearate) solution or zinc sulfate (for zinc stearate) solution is added to an aqueous solution of the sodium stearate. Insoluble zinc or calcium stearate precipitates out of the aqueous solution which contains sodium chloride or sulfate and some excess calcium chloride or zinc sulfate and forms a slurry.
(c) Filtration and washing of the slurry is required to separate the solid stearate as a filter cake from the salt solution. The number of washings and subsequent filtrations required for the separation is determined by the desired finished product purity. Presence of electrolytic salts in the stearate is undesirable in certain applications, e.g., plastic used in electric wire coating or tape.
(d) After the final filtration, the filter cake is dried in ovens or fluid air driers or similar equipment.
(e) Final grinding to obtain the desired particle size is usually accomplished in hammer mills and/or jet mills.
The precipitation process has a number of disadvantages:
(a) Waste water effluent from the filtration and washing steps must be treated before recycling or discharging into waterways. Zinc wastes are particularly objectionable due to their toxicity to aquatic life and degree of difficulty to adequately treat with standard practices, e.g., settling, neutralization, aeration and biodegradation.
(b) Drying the washed filter cake is a high energy consumption unit operation.
(c) It is not practical to reduce the soluble salt content of the finished product to zero.
(d) The number of unit operations in the precipitation process requires high labor input as well as numerous control devices both of which add to the cost and difficulty in quality control.
The following patents are representative of the prior art on dry metallic soap processes.
Begium Pat. No. 644,431--Unilever, describes the production of dry metallic soaps wherein powdered stearic acid and powdered calcium hydroxide were mixed and charged into a laboratory soap press whose extrusion orifice was a perforated plate. After a number of revolutions of the extrusion screw, temperature of the mixture resulting from the compression in the milling in the front part of the extrusion chamber rose suddenly and the saponification product exuded from the press through the plate in the form of vermicelli which cooled rapidly to form a hard, nonfragile product.
U.S. Pat. No. 2,890,232--Rogers, Jr., et al, issued June 9, 1959, describes a batch process wherein stearic acid and calcium oxide were mixed and heated at 200.degree. F.; water was then added to the reaction mixture. Saponification began at 215.degree. F. and was complete at 310.degree. F. The reaction was rapid.
U.S. Pat. No. 3,476,786--Lally et al, issued Nov. 4, 1969, describes a process wherein finely divided water insoluble metallic salts of higher fatty acids were produced by grinding solid fatty acids with a base such as metal oxides, metal hydroxides or mixtures thereof in the presence of a catalyst and in the absence of water. One example employed solid calcium hydroxide and solid stearic acid in the presence of ammonium carbonate as the catalyst.
There is a definite need for a continuous process for preparing dry metallic salts of higher fatty acids.